.border {
    width: 800px;
    height: 500px;
    border: 1px solid red;
    margin: auto;
    display: flex;
}

.h,
.m,
.s {
    display: flex;
    align-items: center;
    justify-content: center;
    flex: 1;
    border: 1px solid red;
    font-size: 120px;
}